1. Field of the Invention
This invention relates to a cured epoxy polymer having improved adhesive properties to a metal deposited on a surface thereof, and more particularly, to a cured epoxy polymer resulting from curing a mixture comprising (a) a diglycidyl ether based upon linoleic dimer acid, (b) an elastomerically modified epoxy resin blend, (c) a diglycidyl ether of bisphenol A (4,4'-isopropylidene diphenol) and/or bisphenol F (methylenediphenol) and (d) a suitable curing agent.
2. Description of the Prior Art
During the past few years, a market for metal-plated polymer parts has grown rapidly as manufacturers have begun to appreciate the functional appearance of such parts when plated with bright, metallic finishes, and to take advantage of economies in cost and weight afforded by substituting molded polymeric parts for metal. Furthermore, such plated finishes are not as susceptible to pitting and corrosion because there is not a galvanic reaction between a polymeric substrate and a plated metal.
Because polymeric materials normally do not conduct electricity, it is common practice to provide a conductive layer or coating, such as copper, by electroless deposition so that an additional thickness of metals, particularly copper, nickel and chromium, can be electrolytically plated onto the electroless copper layer. Electroless deposition refers to an electrochemical deposition of a metal coating on a conductive, nonconductive, or semiconductive substrate in the absence of an external electrical source. While there are several methods of applying this metallic coating by a combined use of electroless and electrolytic procedures, it was not until quite recently that processes were developed which can provide even minimal adhesion of the conductive coating to the polymer. This is because overall adhesion is governed by the bond strength between the polymer substrate and the electroless copper layer. Even with these improved processes, reasonable adhesion can be obtained with only a very few polymers, and then only when great care is taken in all of the steps for the preparation and plating of the polymer substrate.
Polymers extensively employed and upon which electroless deposition is conducted, especially in the printed circuit industry are epoxy polymers resulting from curing uncured diglycidyl ethers of bisphenol A resins produced by a condensation reaction between bisphenol A and epichlorohydrin. The condensation reaction products or epoxy resins have a general structure ##STR1## where n is the number of repeated units in the resin chain. The varying types of these epoxy resins are described in terms of their viscosity or softening point, epoxide equivalent weight and hydroxyl content. The epoxide equivalent weight is defined as the number of grams of resin containing one gram equivalent of epoxide. The epoxide equivalent weight is determinative of the number of repeated units in the epoxy resin chain. The hydroxyl content is defined as the number of equivalents of hydroxyl groups contained in 100 grams of resin.
One of the valuable properties of epoxy resins, i.e., diglycidyl ethers of bisphenol A and variants or modifications thereof, is their ability to transform readily from a liquid or viscous state to tough, hard thermoset solids, i.e., transform from a linear structure to a network crosslinked in three dimensions. This hardening is accomplished by the addition of a chemically active reagent known as a curing agent. Some curing agents promote curing by catalytic action, others participate directly in the curing reaction and are absorbed into the resin chain.
The surface of a cured or crosslinked epoxy article is hydrophobic and is therefore not wet by liquids having a high surface tension. Since electroless depositions using employ aqueous sensitizing and activating solutions having metal ions therein, the surface will not be wet thereby. Since the sensitizing and activating solutions will not wet the surface, the catalytic species are not absorbed onto the surface and subsequent deposition of the metal ions cannot proceed.
In the present state of the art, various methods are available for rendering the surface of a polymer or a plastic material hydrophilic. One method in common practice in plating plastic materials entails mechanical roughening of the surface of the plastic. Initially, this surface roughening is accomplished by some form of mechanical deglazing, such as scrubbing with an abrasive slurry, wet tumbling, dry rolling or abrasive (sand) blasting. However, this prior art method gives an adhesion of up to 3 lbs./in. at 25.degree. C for a copper pattern on a cured polymer based upon the diglycidyl ether of bisphenol A. This adhesion is unsatisfactory for printed circuit boards having a copper pattern thereon. It has been empirically established that a minimum peel strength of about 5 lbs./in., at a 90.degree. peel and a peel rate of 2 in./min. at 25.degree. C is required to prevent a metal coating from blistering or peeling from a plastic surface during variations in temperature and to allow for various processing steps usually employed in generating the copper pattern. The adhesion requirements for printed circuit boards is therefore at least 5 lbs./in. at a 90.degree. peel and a peel rate of 2 in./min. at 25.degree. C (for a copper thickness of 1.4 mil).
In addition, this mechanical deglazing process is costly in that many parts have to be finished by hand and, in the case of relatively small parts, or parts with complex contours, it is very difficult to abrade the surface uniformly by conventional means. Of greatest disadvantage, however, is in forming printed circuits, utilizing a photoimaging process, such as the photoselective metal deposition process revealed in U.S. Pat. No. 3,562,005, assigned to the assignee hereon. The photoimaging process inherently requires a high pattern resolution. This resolution is limited by the topography of the surface on which the pattern is generated. When mechanical deglazing is employed, e.g., by sand blasting, the resolution of the pattern suffers because of the mechanically roughened surface.
In more recent years, chemical deglazing or etching techniques were developed for various plastics using strong acidic solutions. U.S. Pat. No. 3,437,507 reveals a chromic acid treatment of plastics, such as an acrylonitrile-butadiene-styrene (A-B-S) and an amine cured diglycidyl ether of bisphenol A epoxy, to improve the adherence of an electroless deposit to the surface thereof. Again, as indicated above, a minimum adhesion value of 5 lbs./in. has to be met for printed circuit boards. A cured diglycidyl ether of bisphenol A epoxy polymer treated with chromic acid gives adhesion values of about 3 lbs./in. for metallic patterns deposited thereon.
Another method, generally employed for plastics, such as A-B-S, comprises treating the plastic with an organic solvent thereof. U.S. Pat. No. 3,425,946 reveals such a method with A-B-S plastic. However, what solvents are effective depend on the plastic employed and is therefore empirical in nature. Organic solvent pretreatment alone is ineffective in raising the adherence of metallic patterns to cured epoxies such as the epoxy polymers resulting from curing the diglycidyl ether of bisphenol A. A cured diglycidyl ether of bisphenol A epoxy treated in this fashion exhibits an adhesion of about 3 lbs./in., whereas as stated above, 5 lbs./in. is the minimum amount desired for printed circuit boards (at 25.degree. C).
There has not heretofore been electroless metallization of a virgin epoxy, i.e., an as-cured epoxy without pretreatment of any surface thereof with solvents, etchants, abrasives, etc., to render such surface hydrophilic.
A cured epoxy surface which gives improved adherence with respect to an electroless metal deposited thereon, without pretreatment thereof (etching, solvation, abrading, etc.), is therefore needed and is an object of this invention.